I am increasing the service at a commercial retail store from 1200amp to 3000amp. We are installing the new service equipment directly outside from the existing, some of the circuits from the existing will be fed from the new. Can I use the existing panel as a wireway to the new panel if the space in the panel permits? The two panels will be joined by 4inch nipples through the backs.

Among a great number of other things, Section 408.2 requires panelboards to also comply with Article 312.Section 312.8 requires enclosures for OC devices to provide adequate space for conductors feeding through or tapping off by filling the wiring space at any cross section to no more than 40% fill for conductors, and the conductors, splices and taps no more than 75% of the cross sectional area of that space.

If you have the room, then the existing panel can be used to splice extensions into the new panel.